The curtain coating method for the simultaneous coating of multiple layers is well known and is described in U.S. Pat. Nos. 3,508,947 and 3,632,374, which in particular teach the advantages of the method for applying photographic compositions to paper and plastic webs. Thus, these references teach the curtain coating of aqueous gelatin solutions and photographic compositions with viscosities up to and exceeding 100 mPas on photographic substrates. Aqueous gelatin is the usual vehicle for photographic compositions. A major difference between curtain coating and slide bead coating, as taught in U.S. Pat. No. 2,761,791, is that high viscosity compositions can be curtain coated while the bead method fails; consequently, curtain coating offers improved uniformity and reduced drying load for increased productivity with existing dryers. The capability to apply high viscosities arises because the coating composition impinges against the receiving surface at a high speed as a consequence of gravitational acceleration in the free-falling curtain. This impinging flow is sometimes said to provide a hydrodynamic assist for the wetting of the receiving surface.
For a manufacturing process it is desirable to coat at the highest possible speed to maximize productivity from capital equipment. To those skilled in the art of curtain coating, the primary limitations to coating speed are well known (see Liquid Film Coating ed. S. F. Kistler and P. M. Schweizer, Pub. Chapman Hall, 1997). Air entrainment marks the inclusion of air between the coating composition and the receiving surface leading to bubbles or non-uniformities in the coating or both. Puddling refers to the formation of a heel of coating composition at the impingement point of the curtain on the side of the approaching receiving surface. This puddle or heel can be unsteady and so produce a non-uniform coating. Flow recirculations in the heel can trap particles or bubbles and produce a streaked coating. Whether or not particles are trapped, the presence of a heel promotes air entrainment at relatively low speeds as described in the article "Hydrodynamics of Dynamic Wetting" by T. D. Blake, A. Clarke, and K. J. Ruschak, AIChE Journal, Vol. 40, 1994, p. 229. As taught in the article by Clarke in The Mechanics of Thin Film Coatings, ed. P. H. Gaskell et al, World Scientific, 1995, increasing the curtain height, increasing curtain flow rate, and reducing viscosity, separately or in combination, promotes puddling. Coating more layers simultaneously, another way to enhance productivity, promotes puddling by increasing total flow rate.
Various methods have been advanced to postpone air entrainment to higher speeds. Some of these methods take advantage of studies of dynamic wetting showing that lowering viscosity increases air-entrainment speeds. However, in curtain coating, lowering viscosity also promotes puddling, and so anticipating the net result is difficult. In addition, if viscosity is lowered by the addition of solvent, which is usually water for photographic coating compositions, the maximum coating speed for a given drying capacity is reduced.
Many practical coating compositions are non-Newtonian. A Newtonian liquid has a single viscosity value. However, liquids containing high molecular weight polymer or high concentrations of emulsified liquids or dispersed solids typically have a viscosity that decreases with increasing shear rate, the rate of deformation in flow. Such liquids are called shear thinning or pseudoplastic. Typically for such liquids, the viscosity is constant at low shear rates. Above a certain shear rate, viscosity falls as shear rate increases. Ultimately, however, increasing the shear rate leads to the leveling off of viscosity at a value that may be far below that at low shear rates. A standard representation of such behavior is the Carreau model (see for example, "Dynamics of Polymeric Liquids", R. B. Bird, R. C. Armstrong, O. Hassager, Vol. 1 second edition 1987), ##EQU1## where .eta. is the viscosity (mPas) at steady shear rate .gamma.(s.sup.-1), .eta..sub.0 is the constant viscosity (mPas) at low shear rates often referred to as the low-shear viscosity, .eta. is the constant viscosity (mPas) at high shear rates, .lambda. is a time constant (s) and n is the dimensionless power law index. Values for .lambda. and n are obtained by fitting viscosity measurements of the liquid to Equation 1. For a Newtonian liquid, n equals 1, and for shear-thinning liquids n is less than 1; the smaller that n is, the more rapidly viscosity falls with increasing shear rate.
To obtain high coating speeds, U.S. Pat. No. 5,391,401 to Blake et al. teaches an optimum rheological profile, by which is meant an optimum relationship between viscosity and shear rate. The optimum rheological profile for curtain coating provides a low viscosity at the shear rates expected near the dynamic wetting line, where the coating composition wets the receiving surface, and a high viscosity at the much lower shear rates expected in all other parts of the flow. A low viscosity at the wetting line promotes high speeds without air entrainment, while the higher viscosity elsewhere reduces the propensity for puddling and promotes the delivery and drying of uniform layers. However, highly shear-thinning coating compositions require coating dies custom designed for uniform distribution across the width of the coating, whereas for slightly shear thinning coating compositions, general purpose dies may be used. Gelatin, the primary binder for photographic products, is slightly shear thinning, and so highly shear-thinning coating compositions depend upon the presence of other components, such as polymeric thickening agents or concentrated colloids. Moreover, the amount of gelatin required by the formulation can limit the extent of shear thinning. It can therefore be difficult to obtain a specific rheological profile while maintaining the product-specific properties of a coating composition.
A method to increase speeds has been taught in EP 0563308 to Blake and Ruschak whereby air entrainment is postponed to higher speeds while suppressing puddling. In this method the direction of movement of the receiving surface is angled with respect to the plane of the curtain such that the curtain forms an acute angle with the approaching receiving surface, and high curtains are used for hydrodynamic assist of dynamic wetting. The geometric change reduces the propensity for puddling and thereby allows advantage to be taken of both a high impingement speed and a shear-thinning coating composition to increase coating speed. However, the speed increase by this method is limited by the achievable low level of viscosity of the coating composition at high shear rates.
In other methods, forces are applied, such as by an electrostatic or magnetic field, to postpone air entrainment to higher coating speeds. The creation of an electrostatic field at the impingement point to increase speeds in curtain coating is taught in WO 89/05477 to Hartman. However, this method can be limited by puddling when used in conjunction with high flow rate or low viscosity.
Another method to alleviate the problems of puddling and air entrainment is taught in U.S. Pat. No. 5,393,571 to Suga et al. In this method, coating compositions with high viscosity at low rates of shear, around 10 s.sup.-1, are applied to a receiving surface of significant roughness using curtain coating. The method applies to flow rates above 4 cc/s per cm of coated width, a nominal roughness of the receiving surface exceeding 0.3 microns, a low-shear viscosity of a coating composition exceeding 90 mPas, an average viscosity for all layers exceeding 80 mPas, and coating speeds exceeding 325 m/min. There exist several standard measures, R.sub.a, R.sub.z, R.sub.max, etc. (see DIN4768, ISO4287, BS1134), for specifying surface roughness relating to different phenomena. For example, R.sub.a =0.3 .mu.m and R.sub.z =0.3 .mu.m specify significantly different surfaces. Furthermore, R.sub.a and R.sub.z can give numerical values differing by an order of magnitude for the same surface. Thus the roughness values specified for the method of Suga et al. are nominal and do not unequivocally identify applicable surfaces. Many substrates for photographic products, and likely all paper substrates, ostensibly meet this nominal roughness requirement. Suga et al. teach increasing the viscosities of coating compositions for the purposes of their method by the addition of a thickening agent that interacts with the binder in the composition, i.e. gelatin, to increase the viscosity at low shear rate without substantially increasing its viscosity at high shear rate, the implication being that a high viscosity at high shear rates is disadvantageous. However, thickening agents added to photographic compositions can cause interactions with other components that adversely affect the product. Insolubility is an example of an adverse chemical interaction, and degraded hardness and sensitometric response are examples of adverse performance interactions.
In view of increasing demands for productivity, there is need for a high-speed curtain-coating method negating the limitations of puddling and air entrainment. Such a method should have latitude for accommodating a wide range of viscosity because of the practical problems of achieving high viscosity in all cases. The range of viscosity latitude should preferably extend to high viscosity obtained through reducing volatile components such as water in order to reduce drying load and so obtain higher coating speeds on the same manufacturing equipment.